Xue-Zhong Sun

Photoreactivity examined through incorporation in metal−organic frameworks

Metal-organic frameworks, typically built by bridging metal centres with organic linkers, have recently shown great promise for a wide variety of applications, including gas separation and drug delivery. Here, we have used them as a scaffold to probe the photophysical and photochemical properties of metal-diimine complexes. We have immobilized a M(diimine)(CO)(3)X moiety (where M is Re or Mn, and X can be Cl or Br) by using it as the linker of a metal-organic framework, with Mn(II) cations acting as nodes. Time-resolved infrared measurements showed that the initial excited state formed on ultraviolet irradiation of the rhenium-based metal-organic framework was characteristic of an intra-ligand state, rather than the metal-ligand charge transfer state typically observed in solution, and revealed that the metal-diimine complexes rearranged from the fac- to mer-isomer in the crystalline solid state. This approach also enabled characterization of the photoactivity of Mn(diimine)(CO)(3)Br by single-crystal X-ray diffraction.

A combined theoretical and experimental study on the role of spin states in the chemistry of Fe(CO)5 photoproducts.

A combined experimental and theoretical study is presented of several ligand addition reactions of the triplet fragments (3)Fe(CO)(4) and (3)Fe(CO)(3) formed upon photolysis of Fe(CO)(5). Experimental data are provided for reactions in liquid n-heptane and in supercritical Xe (scXe) and Ar (scAr). Measurement of the temperature dependence of the rate of decay of (3)Fe(CO)(4) to produce (1)Fe(CO)(4)L (L = heptane or Xe) shows that these reactions have significant activation energies of 5.2 (+/-0.2) and 7.1 (+/-0.5) kcal mol(-1) respectively. Nonadiabatic transition state theory is used to predict rate constants for ligand addition, based on density functional theory calculations of singlet and triplet potential energy surfaces. On the basis of these results a new mechanism (spin-crossover followed by ligand addition) is proposed for these spin forbidden reactions that gives good agreement with the new experimental results as well as with earlier gas-phase measurements of some addition rate constants. The theoretical work accounts for the different reaction order observed in the gas phase and in some condensed phase experiments. The reaction of (3)Fe(CO)(4) with H(2) cannot be easily probed in n-heptane since conversion to (1)Fe(CO)(4)(heptane) dominates. scAr doped with H(2) provides a unique environment to monitor this reaction--Ar cannot be added to form (1)Fe(CO)(4)Ar, and H(2) addition is observed instead. Again theory accounts for the reactivity and also explains the difference between the very small activation energy measured for H(2) addition in the gas phase (Wang, W. et al. J. Am. Chem. Soc. 1996, 118, 8654) and the larger values obtained here for heptane and Xe addition in solution.

2,5‐Bis(p‐R‐arylethynyl)rhodacyclopentadienes Show Intense Fluorescence: Denying the Presence of a Heavy Atom

Photophysikalische Untersuchungen ergaben fur eine Reihe von Rhodiumkomplexen unerwartet hohe Fluoreszenzquantenausbeuten (Φf bis 69 %, τf≈1–3 ns) und ein unerwartet langsames Intersystem-Crossing. Diese neue Verbindungsklasse hinterfragt das gangige Modell zum Verhalten angeregter elektronischer Zustande und zur Rolle von Schweratomen in Intersystem-Crossing-Prozessen. THF=Tetrahydrofuran, Tol=Toluol.

Complete Family of Mono-, Bi-, and Trinuclear ReI(CO)3Cl Complexes of the Bridging Polypyridyl Ligand 2,3,8,9,14,15-Hexamethyl-5,6,11,12,17,18-hexaazatrinapthalene: Syn/Anti Isomer Separation, Characterization, and Photophysics

The syn and anti isomers of the bi- and trinuclear Re(CO)(3)Cl complexes of 2,3,8,9,14,15-hexamethyl-5,6,11,12,17,18-hexaazatrinapthalene (HATN-Me(6)) are reported. The isomers are characterized by (1)H NMR spectroscopy and X-ray crystallography. The formation of the binuclear complex from the reaction of HATN-Me(6) with 2 equiv of Re(CO)(5)Cl in chloroform results in a 1:1 ratio of the syn and anti isomers. However, synthesis of the trinuclear complex from the reaction of HATN-Me(6) with 3 equiv of Re(CO)(5)Cl in chloroform produces only the anti isomer. syn-{(Re(CO)(3)Cl)(3)(μ-HATN-Me(6))} can be synthesized by reacting 1 equiv of Re(CO)(5)Cl with syn-{(Re(CO)(3)Cl)(2)(μ-HATN-Me(6))} in refluxing toluene. The product is isolated by subsequent chromatography. The X-ray crystal structures of syn-{(Re(CO)(3)Cl)(2)(μ-HATN-Me(6))} and anti-{(Re(CO)(3)Cl)(3)(μ-HATN-Me(6))} are presented both showing severe distortions of the HATN ligand unit and intermolecular π stacking. The complexes show intense absorptions in the visible region, comprising strong π → π* and metal-to-ligand charge-transfer (MLCT) transitions, which are modeled using time-dependent density functional theory (TD-DFT). The energy of the MLCT absorption decreases from mono- to bi- to trinuclear complexes. The first reduction potentials of the complexes become more positive upon binding of subsequent Re(CO)(3)Cl fragments, consistent with changes in the energy of the MLCT bands and lowering of the energy of relevant lowest unoccupied molecular orbitals, and this is supported by TD-DFT. The nature of the excited states of all of the complexes is also studied using both resonance Raman and picosecond time-resolved IR spectroscopy, where it is shown that MLCT excitation results in the oxidation of one rhenium center. The patterns of the shifts in the carbonyl bands upon excitation reveal that the MLCT state is localized on one rhenium center on the IR time scale.

Energy dispersive XAFS: characterization of electronically excited states of copper(I) complexes.

Energy dispersive X-ray absorption spectroscopy (ED-XAS), in which the whole XAS spectrum is acquired simultaneously, has been applied to reduce the real-time for acquisition of spectra of photoinduced excited states by using a germanium microstrip detector gated around one X-ray bunch of the ESRF (100 ps). Cu K-edge XAS was used to investigate the MLCT states of [Cu(dmp)2]+ (dmp =2,9-dimethyl-1,10-phenanthroline) and [Cu(dbtmp)2]+ (dbtmp =2,9-di-n-butyl-3,4,7,8-tetramethyl-1,10-phenanthroline) with the excited states created by excitation at 450 nm (10 Hz). The decay of the longer lived complex with bulky ligands, was monitored for up to 100 ns. DFT calculations of the longer lived MLCT excited state of [Cu(dbp)2]+ (dbp =2,9-di-n-butyl-1,10-phenanthroline) with the bulkier diimine ligands, indicated that the excited state behaves as a Jahn–Teller distorted Cu(II) site, with the interligand dihedral angle changing from 83 to 60° as the tetrahedral coordination geometry flattens and a reduction in the Cu–N distance of 0.03 Å.

Photoinduced N2 loss as a route to long-lived organometallic alkane complexes: A time-resolved IR and NMR study

Photolysis of CpRe(CO)2(N2) in cyclopentane or 2,2-dimethylbutane with a UV lamp via a quartz fibre inserted into the NMR probe allows generation of CpRe(CO)2(cyclopentane) and CpRe(CO)2(2,2-dimethylbutane). The latter is observed in three isomeric forms according to the site of co-ordination to the rhenium. The major isomer, CpRe(CO)2(2,2-dimethylbutane-η2-C1,H1), exhibits a 1H NMR resonance for the co-ordinated hydrogen at δ = −2.19 with 1JC–H = 118 Hz. The photochemistry of Cp‡Re(CO)2(N2) (Cp‡ = η5-1,2-C5H3(tBu)2) in alkane solution is also reported. Two new organometallic alkane complexes, Cp‡Re(CO)2(alkane) (alkane = cyclopentane, n-heptane) have been characterized by IR spectroscopy following irradiation of Cp‡Re(CO)2(N2) and their rate constants for reaction with CO have been determined. The reaction with cyclopentane has also been studied by NMR spectroscopy at 190 K with in situ laser irradiation at 355 nm. Cp‡Re(CO)2(c-C5H10) is shown to exhibit the characteristic features of an alkane complex in the NMR spectrum, viz. a large isotopic shift of the 1H resonance at δ = −2.44 upon partial deuteration of the alkane (Δδ = 1.77 ppm), a large 1JC–H (114 Hz) and a large negative 13C chemical shift (δ = −33.8). We find no evidence for CO loss or agostic interactions of the t-butyl groups under these conditions. Cp‡Re(CO)2(alkane) has a slightly shorter lifetime (ca. 5x) than CpRe(CO)2(alkane) for a given alkane. Photolysis of CpRe(CO)2(N2) to form the organometallic alkane complex occurs with a much higher yield than for CpRe(CO)3. Efficient photo-ejection of N2 from Cp‡Re(CO)2(N2) is observed upon either 266 or 355 nm laser irradiation. A dinitrogen precursor allows for the use of longer wavelength irradiation and the generation of a higher concentration of the alkane complex following each laser pulse.

Understanding the factors affecting the activation of alkane by Cp′Rh(CO)2 (Cp′ = Cp or Cp*)

Fast time-resolved infrared spectroscopic measurements have allowed precise determination of the rates of activation of alkanes by Cp′Rh(CO) (Cp′ = η5-C5H5 or η5-C5Me5). We have monitored the kinetics of C─H activation in solution at room temperature and determined how the change in rate of oxidative cleavage varies from methane to decane. The lifetime of CpRh(CO)(alkane) shows a nearly linear behavior with respect to the length of the alkane chain, whereas the related Cp*Rh(CO)(alkane) has clear oscillatory behavior upon changing the alkane. Coupled cluster and density functional theory calculations on these complexes, transition states, and intermediates provide the insight into the mechanism and barriers in order to develop a kinetic simulation of the experimental results. The observed behavior is a subtle interplay between the rates of activation and migration. Unexpectedly, the calculations predict that the most rapid process in these Cp′Rh(CO)(alkane) systems is the 1,3-migration along the alkane chain. The linear behavior in the observed lifetime of CpRh(CO)(alkane) results from a mechanism in which the next most rapid process is the activation of primary C─H bonds (─CH3 groups), while the third key step in this system is 1,2-migration with a slightly slower rate. The oscillatory behavior in the lifetime of Cp*Rh(CO)(alkane) with respect to the alkane’s chain length follows from subtle interplay between more rapid migrations and less rapid primary C─H activation, with respect to CpRh(CO)(alkane), especially when the CH3 group is near a gauche turn. This interplay results in the activation being controlled by the percentage of alkane conformers.

A systematic approach to the generation of long-lived metal alkane complexes: combined IR and NMR study of (Tp)Re(CO)2(cyclopentane)

Short wavelength photolysis of (Tp)Re(CO)(3) (Tp = tris(pyrazol-1-yl)borate) at low-temperature in cyclopentane yielded (Tp)Re(CO)(2)(cyclopentane), an alkane complex with three nitrogen ligands that was characterised by NMR spectroscopy.

Modification of coordination networks through a photoinduced charge transfer process

A metal-bearing coordination network synthesised from Re(2,2′-bipyridine-5,5′-dicarboxylate)(CO)3Cl bridging ligands and Cu(II) nodes, [{Cu(DMF)(H2O)[LRe(CO)3Cl]}·DMF]∞ReCu, undergoes an irreversible photoinduced charge transfer process. We demonstrate using time-resolved IR spectroscopy the nature of this photoinduced process and how, under suitable conditions, it is possible to initiate irreversible modification of the crystal through induction of the charge transfer process. As a result we are able to use the photoinduced process, which arises purely as a result of the structure of the coordination network, to write on crystals.

Study of the reaction of Rh(acac)(CO)2 with alkenes in polyethylene films under high-pressure hydrogen and the Rh-catalysed hydrogenation of alkenes

Abstract The thermal reaction of Rh(acac)(CO) 2 with alkenes has been studied both in the absence and in the presence of high-pressure hydrogen using in situ FTIR and polymer matrix techniques. A series of rhodium alkenes complexes, Rh(acac)(CO)(alkene) (alkene=ethene, propene, 1-butene, 1-octene and trans -3-octene), have been characterized using IR spectroscopy. In the presence of a high-pressure hydrogen, catalytic hydrogenation of alkenes was achieved using Rh(acac)(CO) 2 within the polyethylene matrix. These results suggest that this hydrogenation process follows the so-called “olefin route” operating via a sequence of ligand loss, binding of an alkene, oxidative addition of hydrogen to the rhodium metal centre, insertion of the coordinated alkene into the MH bond and finally reductive elimination of the alkane. Rh(acac)(CO) appears to be the active catalytic species in this process. High-pressure polymer matrix techniques have allowed us to unravel some of the interconversions of the catalytic species involved in this catalytic process.